Blender Git Loki

Fix T91405: Block artefacts in WEBM video

Issue was caused by incorrect FFmpeg asynchronous decoding API. In most
cases, decoder returns 1 frame each time it is fed by 1 packet. Here
decoder wanted to return more frames, but our code always expected only
one.

Before sending new packets to decoder, check if there are frames to
receive. If there are, process them, otherwise continue decoding as
usual.

Reviewed By: zeddb, sergey

Differential Revision: https://developer.blender.org/D13079